1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a printer, a facsimile machine, etc., and more particularly relates to an image forming apparatus including an image transfer device having a transfer roller.
2. Discussion of the Background
In an image forming apparatus utilizing electrophotography, such as a copying machine, a printer, a facsimile machine, etc., an image transfer device is provided to transfer a toner image formed on an image carrier of the apparatus to a transfer sheet conveyed to a transfer position between the image carrier and the transfer device.
An example of an image transfer device includes a charger to apply a corona charge to a transfer sheet when transferring an image on the image carrier to the transfer sheet. Another example of an image transfer device includes a transfer roller which transfers an image formed on an image carrier to a transfer sheet by applying a voltage to the transfer roller when the transfer sheet passes through a transfer position between the transfer roller and the image carrier.
In such a transfer device including a transfer roller, since a surface of the transfer roller contacts a surface of the image carrier via a transfer sheet, toner on the surface of the image carrier is hardly transferred to the surface of the transfer roller. However, when multiple images are formed in succession on the image carrier, such as when making multiple copies, since a gap exists between the images, toner adhering to a part of the image carrier corresponding to the gap is transferred to the surface of the transfer roller. Further, when a transfer sheet to be conveyed to the transfer position jams and is not conveyed to the transfer position, a toner image formed on the surface of the image carrier directly contacts the surface of the transfer roller, and as a result the toner is put on the surface of the transfer roller.
If a next toner image is then formed to be transferred to a next transfer sheet, when the next transfer sheet passes the transfer position for the transfer, such toner put on the surface of the transfer roller is transferred to a back surface of the next transfer sheet. When a duplex copy is made, after an image is transferred to a first surface of a transfer sheet, the transfer sheet carrying the transferred image on the first surface is returned so that another toner image is transferred to the back surface. Therefore, if the back surface of the transfer sheet is stained by toner remaining on the surface of the transfer roller when transferring the image to the first surface of the transfer sheet, the image transferred to the back surface next is disturbed by such toner transferred from the transfer roller. Further, since the first surface of the transfer sheet on which the image is first transferred contacts the transfer roller, the image on the first surface is also damaged by the toner remaining on the transfer roller if any toner remains on the transfer roller.
Therefore, in background image forming apparatuses, a cleaning device is provided to scrape off toner adhering to a surface of a transfer roller, such as, for example, a cleaning blade made of rubber or the like. Toner is scraped off the transfer roller by bringing an edge of the cleaning blade in contact with the surface of the transfer roller which is rotating.
Further, a mechanism is provided to separate the cleaning blade from the transfer roller when cleaning is not performed, for preventing damage to the surface of the transfer roller.
Further, in order to achieve optimum cleaning performance of the cleaning blade, a control mechanism is provided to control a pressing force to press the cleaning blade against the surface of the transfer roller.
However, even when such a cleaning device is provided for cleaning the surface of the transfer roller, a surface of the transfer roller is not cleaned sufficiently when an amount of toner adhering to the surface of the transfer roller is large. For example, when an image is formed in a full color, an amount of toner applied to the image for development is large compared to a case in which the image is formed in a mono-color, and consequently toner adhering to the surface of the transfer roller increases. Therefore, the toner adhering to the surface of the transfer roller is not completely removed by the cleaning device.
Further, when a certain type of transfer sheet is used, a problem occurs that paper dust of the transfer sheet adheres to the surface of the transfer roller. When the transfer device uses a transfer roller, since the transfer sheet is pressed by the transfer roller to the image carrier and is rubbed by the transfer roller, the transfer sheet produces paper dust. Therefore, when the transfer sheet contains a relatively large amount of calcium carbonate, for example, the paper dust coming off the transfer sheet contains calcium carbonate. Such paper dust containing calcium carbonate adheres to the surface of the transfer roller, forming a film thereupon. Such a film can not be easily scraped off by the cleaning device even when the pressing force pressing the cleaning device against the transfer roller is optimized. If the film remains on the surface of the transfer roller and toner is put on the film, the toner adheres to the film and can not be easily removed from the film.